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Local Geology: Wallace Creek

As you may have noticed, I was on quite a hiatus during this holiday season. Way too much going on. Hopefully when I start school next week I’ll settle back into my old routine. But now that I am back to writing blog posts, what shall I write about to not only break the silence but to also kick off the new year? The answer came to me while I was at Carrizo Plain National Monument this weekend: Wallace Creek.

Wallace Creek may seem like nothing special. Hell most of the year it’s just a creek bed. It doesn’t have any of that lush, picturesque vegetation that you find around creeks and streams. And yet Wallace Creek often shows up in discussions on geology and has even been featured on the History Channel program “How the Earth was made”. Here’s a clue to why Wallace Creek is of such renown: look at the photo below and see if you notice anything odd about it:

What is so odd about Wallace Creek?

If you look carefully, you can see that the creek bed starts off straight, but then veers off away from where you are and then veers off back towards the plain. If you still can’t see it (I was holding my camera as high as I could), then this aerial photo ought to help:

Aerial view of Wallace Creek

There you can not only see the funky bend but also another bend to the left. That is an ancient bend that the creek once flowed through but has since run dry because it was cut off from the main channel. This is all because Wallace Creek runs across the San Andreas Fault. For those of you who don’t know, the San Andreas Fault is an 800 mile fault line that runs north to south in California and is one of the longest faults in North America. San Andreas is what is known as a right lateral fault. Lateral movement occurs when rocks on one side of the fault slide past rocks on the other side in a horizontal fashion with little (if any) vertical movement. Right lateral means the direction its going: if you and a buddy stood on opposite sides of the fault and faced each other, you would be moving to your buddy’s right and vice versa.

But how did the creek get its shape? While the fault moves at about 1.2 inches a year Wallace Creek did not take its odd shape (called an offset channel) gradually over time as might be suggested by this fact. Rather, its shape came about very suddenly. Instead of moving gradually over hundreds or even thousands of years, Wallace Creek has been twisted by jumps made during earthquakes. Look back at the aerial photograph. See the channel on the left? It’s what’s called a beheaded channel. It was once the main creek bed. It flowed straight across the fault around 10,000 years ago but a series of earthquakes every few hundred years offset the channel. Eventually the creek cut through the first bend and separated the offshoot and began carving the modern channel about 3800 years ago. More earthquakes offset the new creek creating the offset channel we see today. The ancestral channel is but a ghost; a stream without a source. Wallace Creek may be the largest and most dramatic example, but right next to it are several smaller streams that have also become offshoot and beheaded channels.

So if Wallace Creek moves in leaps and bounds during earthquakes, when was the last time it moved? History can help us with this one. Farmers and ranchers have been working on Carrizo Plain since the time of the Spanish. But they had no idea that they were sitting on a time bomb. On January 9, 1857, the land seemed to come alive as it shook violently. San Andreas had unleashed an earthquake just north of Carrizo Plain that caused Wallace Creek as well as the rest of the valley to offset by 30 feet. The earthquake was one of the most powerful in American history: an estimated 8.0. The quake was felt from Marysville south to San Diego and east to Las Vegas, Nevada. The nearby Kern River had its current turned upstream and its waters ran four feet deep over its banks. A cattleman on the Plain reported that his circular sheep pen was converted into a crude s-shape. The surface of the earth along the fault was ruptured for almost 220 miles.

Because of the arid climate of Carrizo Plain most of the evidence of the restless San Andreas Fault has been preserved. Not only can you see the many stream channels that so dramatically demonstrate the movement of the earth, but also the fault itself. Since there has been little rain to cause erosion, you can still see the rift that San Andreas left like a great scar on the land:

View of Carrizo Plain from atop Caliente Ridge. The San Andreas Fault is the short ridge just below the mountains

Wallace Creek is a fascinating look into the workings of the earth. Had it not been for the San Andreas Fault, it would be nothing but a barren, unexciting gash in the land. But Wallace Creek is just one of the reasons I have become so fond of Carrizo Plain. I hope to one day show you what an incredible place it is. But for now, I’d like you to take a moment to ponder on the marvels of the Sand Andreas Fault at a little place called Wallace Creek.